The number of Republicans running against Missouri Sen. Claire McCaskill, who is widely considered among the most vulnerable Democrats up for re-election in 2012, could soon double as she looks to move past recent ethics questions.

Two Republicans are already in, and two more are taking serious looks at the race, which the GOP sees as increasingly winnable. Former Ambassador to Luxembourg Ann Wagner met with the National Republican Senatorial Committee last week to discuss her interest, and Rep. Todd Akin is actively exploring a bid.

The entrance of the former state party chairwoman and the six-term Congressman would further crowd a field that already includes former state Treasurer Sarah Steelman and 2010 House candidate Ed Martin.

As the most highly recognizable name among the four and likely able to raise the most money, Akin, who received a perfect score for the second consecutive year in the American Conservative Union vote ratings, would likely jump to the front of the pack should he decide to run.

“If Todd Akin gets in the race, he owns the field,” a Missouri-based Republican strategist said. “He’s got 100 percent name ID in the most expensive media market. ... He’s the most conservative candidate. You can’t get to the right of him.”

Akin, who represents the 2nd district, Wagner and Martin are all from the St. Louis area.

Wagner told Roll Call that she is keeping her options open and that Akin’s decision will have an effect on what she decides to do. “Running for Senate is my preference,” she said, but added that she could be open to running for Akin’s seat.

While in Washington, D.C., last week, Wagner attended a Crossroads GPS event at Johnny’s Half Shell, as did Senate Minority Leader Mitch McConnell (R-Ky.) and NRSC Chairman John Cornyn (Texas).

“Claire McCaskill is beatable and out of step with Missourians,” said Wagner, who served as President George W. Bush’s ambassador to Luxembourg. “The NRSC and much of our Republican leadership feels the same way.”

McCaskill faced an uphill challenge even before her Senate office said she would repay $88,000 spent on charter flights through a company her family partly owns. She barely won her seat in 2006, ousting Republican Sen. Jim Talent, who took a pass at a rematch in 2012. Missouri Democrats have seen their ranks depleted in recent elections, and the 2010 Democratic Senate candidate was trounced by now-Sen. Roy Blunt. (Wagner was chairwoman of Blunt’s campaign.)

Democrats say not to underestimate McCaskill, but Republicans have been more positive about the race than ever since the charter issue surfaced.

Republicans in Missouri and Washington, D.C., quickly jumped on the news, which was first reported by Politico, to knock the first-term Senator, whom they believe to be in increasing political peril.

“Republicans have long believed that Sen. McCaskill’s unabashed support for the Obama spending agenda made her uniquely vulnerable, but interest in this race has only grown as McCaskill’s ethics problems have mounted in recent days,” NRSC spokesman Brian Walsh said.

“There are several strong Republican candidates and potential candidates looking at the Missouri Senate race, and as with everywhere else, we’re happy to meet with them,” Walsh added.

Democrats defend McCaskill as one of the strongest advocates for good government and say re-election prospects will not take a hit by the report.

“No one has fought harder to protect taxpayers and hold government accountable than Claire,” said Matt Canter, spokesman for the Democratic Senatorial Campaign Committee. “National Republicans know that they have a very bloody primary brewing in Missouri with a number of deeply flawed candidates, and the only thing they can do is to try and distract from that.”

“I don’t think it’s going to be a make-or-break moment for the U.S. Senator,” one Democratic operative in Missouri said. “Even in what could be a tough year, I think Claire is in a good position given who is going to run against her.”

In 2010, Martin nearly beat Rep. Russ Carnahan (D) in a St. Louis-area district that President Barack Obama won two years earlier with 60 percent of the vote. He launched a new website — AirClaire.com — six hours after the news first broke about McCaskill’s charter plane issue. He said it has already paid off with several checks from donors.

Martin told Roll Call that Missourians are looking for a “new wave of conservatives” who are not interested in becoming career politicians. He said he is running because of the support that he received following his House bid last year, but Martin conceded that McCaskill would most likely outspend him and receive extensive third-party support.

“Claire is going to have as much money as she needs, and she already has public-sector unions running radio ads in Missouri saying she’s great,” Martin said. “The kitchen sink will be thrown in here to save a Democratic seat. Although she’s wounded politically, I don’t have any illusions that Claire is going to be easy to beat here.”

A poll released last week by Democratic firm Public Policy Polling found Steelman leading a four-way primary with 31 percent, followed by Akin with 24 percent, Martin with 9 percent and Wagner with 2 percent. The same firm, in a poll conducted before news broke of McCaskill’s charter plane use, found each Republican within single digits of the Senator.

Messages left with the Steelman campaign and Akin’s Congressional office were not returned by press time.

Republican operatives in the state said the map has changed since 2008, when Obama barely lost Missouri despite spending significant resources on the former bellwether state. They predicted his numbers will not likely improve from that performance.

An early March PPP poll found Obama with a 43 percent approval rating in Missouri, lower than his national average.

McCaskill has sought to highlight her independence from the national party, but Republicans will continue to bring up her votes in favor of big-ticket items such as the economic stimulus and health care.

“Obama is clearly going to lose Missouri, regardless of who his opponent is,” one GOP strategist said. “For her to overcome him — she’s got a ceiling now against almost any Republican.”